The major histocompatibility complex (MHC) Class I antigens are expressed on virtually all types of vertebrate cells examined. These highly polymorphic transmembraneous glycoproteins have a 45 kD heavy chain consisting of a short cytoplasmic C-terminal tail, a transmembraneous region, and an extracellular N-terminal sequence which encompasses three domains, .alpha..sub.1 - and .alpha..sub.2 -domains carry all the immunological polymorphism, while the membrane-proximal .alpha..sub.3 is non-covalently associated with the 12 kD .beta..sub.2 microglobulin.
The MHC Class I antigen plays an essential role in restriction of the target cell repertoire of cytoxic T-lymphocytes (CTL), which involves preferential utilization of the different polymorphic MHC Class I antigens, H-2K, -D or -L (for mouse) or HLA-A, -B, or -C (for human), e.g. in recognition of viral infected cells. For the most part, attention has been directed to the role of the MHC Class I antigens in restricting T-cell activity. However, some authors have suggested a broader role for the antigens, which will be discussed below.